1. Field of the Invention
The present invention relates to a reflective original, exposure method, and device manufacturing method.
2. Description of the Related Art
At present, the manufacture of semiconductor devices such as a DRAM and MPU has been studied and developed to implement devices having a line width of 22 nm or less in design rule. An exposure apparatus using extreme ultraviolet light (EUV) is prominent as an exposure apparatus used in this generation. An exposure apparatus generally irradiates, with light, a mask (also called an original or reticle) on which a circuit pattern is drawn. The exposure apparatus then reduces the image of the circuit pattern to, for example, ¼ by using a projection optical system, and projection-exposes the reduced image to a wafer coated with a photosensitive agent (resist). In actual exposure, if a particle is attached to the circuit pattern surface of a mask, the image of the particle is transferred at identical positions in respective shots. This decreases the semiconductor device manufacturing yield.
To solve this problem, a transparent protection film called a pellicle is added to a mask in a conventional exposure apparatus having, as a light source, a g-ray, i-ray, KrF laser, ArF laser, or the like. The circuit pattern is protected from a particle by arranging the pellicle at an interval of only several mm from the circuit pattern surface. A particle attached to the pellicle is defocused from the object plane serving as the circuit pattern surface. In general, therefore, a particle is not transferred as an image on a wafer as long as it is a predetermined size or smaller.
However, in an EUV exposure apparatus to which the present invention is applied, there is no practical pellicle for EUV light. To satisfy a requested transmittance, the thickness of the pellicle needs to be about several ten nm. However, with this pellicle thickness, the pellicle cannot be configured at a satisfactory strength in both the mechanical aspect upon a change between the atmospheric pressure and the ambient pressure in a vacuum environment when a mask is transferred, and the thermal aspect when EUV light is absorbed and the temperature rises.
In the EUV exposure apparatus, the mask inevitably becomes pellicle-less. If a particle is generated in the exposure apparatus, it may be attached to the circuit pattern surface of the mask. For example, a case in which a device with a 22-nm design rule is manufactured will be examined. If a 0.1-μm particle is attached to the circuit pattern of a mask and the reduction magnification of the projection optical system is 4:1, the particle image becomes 25 nm on a wafer. Hence, no device can be manufactured. In practice, the size of a particle to be managed is smaller and is equal to or smaller than several ten nm.
The cause of generation and behavior of a particle of a nanometer size generated in a vacuum apparatus have not been fully clarified. However, a particle generated by sliding or friction in the operation of a reticle stage, robot hand, or gate valve, or the like, debris slightly flying from the light source side, and the like are conceivable as generation causes. A reticle is loaded and unloaded via a load lock chamber. When the inside of the load lock chamber is evacuated or released to the air, a particle is generated. Thus, the management of a particle on an EUV mask is a critical issue. To solve this, mask cleaning and particle inspection are essential.
As an EUV mask cleaning method, various methods such as wet cleaning and dry cleaning are proposed. In most cases, a particle is physically attached inside the EUV exposure apparatus, as described above. The dry cleaning method is also effective, so US Patent Application Publication No. 2006/0072085 has proposed cleaning using a laser shock wave, and Japanese Patent Laid-Open No. 2000-088999 has proposed dry laser cleaning.
In EUV mask inspection, to inspect a particle of several ten nm or less, the wavelength of the light source of the inspection apparatus is becoming shorter and its output is becoming larger. For example, 488 nm is used as the light source wavelength. Recently, laser light sources having wavelengths of 266 nm, 248 nm, and 193 nm are being used. Inspection apparatuses using these light sources detect a particle by performing die to die inspection (comparing images of adjacent dies) which has widely been used in conventional mask inspection. Alternatively, the inspection apparatuses detect a particle by performing die to database inspection (comparison inspection of an image with design data), and use the detection result for management of a particle on an EUV mask.
US Patent Application Publication No. 2006/0072085 has proposed a cleaning apparatus which condenses a laser beam in a space near a cleaning target surface to generate a shock wave, and a particle on the surface is removed by the pressure wave. This apparatus is a dry cleaning apparatus and is applicable in situ inside a semiconductor exposure apparatus. Generally, shock wave cleaning often uses a Q-switched Nd:YAG laser as the light source. The YAG laser is a pulse laser having a wavelength of 1,064 nm, a pulse width of about 10 ns, and energy of several hundred mJ or more per pulse. The optical axis is arranged parallel to a cleaning target surface, and a laser beam is condensed to generate a shock wave. To efficiently remove a particle, the distance between the laser spot position and the surface is about several mm. On the surface immediately below a laser condensing unit, thermal damage may be generated by high energy of the pulse laser.
Japanese Patent Laid-Open No. 2000-088999 has disclosed a technique of removing, by a laser beam, a contaminant on the surface of a reflection optical element arranged in an exposure apparatus. The technique in Japanese Patent Laid-Open No. 2000-088999 is general laser cleaning. A cleaning target surface is cleaned by a thermal action of directly irradiating the cleaning target surface with a laser beam of a short wavelength (for example, 248 nm) to excite thermal vibrations of the surface and a particle, or a photochemical action of directly breaking chemical bonds of a contaminant. Japanese Patent Laid-Open No. 2000-088999 has disclosed an example in which an EUV optical element having a multilayer film structure is cleaned by laser cleaning. In this case, the number of layers of the multilayer film is increased, and even removal of the top of the multilayer film by the laser does not influence the EUV reflectance. However, when this technique is applied to an EUV mask, a removed multilayer film is attached again as a foreign substance (particle) to the mask surface.
Similarly, thermal damage may occur even during particle inspection of an EUV mask. This is because the foreign substance detection sensitivity of the inspection apparatus needs to be increased as the size of a foreign substance decreases. In general, the amount of light scattered from a foreign substance upon irradiation with light is inversely proportional to the fourth power of the wavelength and proportional to energy. To increase the detection sensitivity, it is necessary to shorten the wavelength of the light source and increase the energy of the incident light. In this case, the multilayer film greatly absorbs a laser beam to raise the temperature and decrease the reflectance of EUV light.
In this manner, the surface may be damaged by heat in both the EUV mask cleaning and inspection methods. The surface of an EUV mask has a multilayer film, and this structure is sensitive to damage in cleaning and inspection. Therefore, the structure of the multilayer film is desirably durable to heat.